Returns a list of all purchase transactions in the administration.
The filter argument allows you to filter the list of invoices. Filters are a combination of keys and values,
separated by a comma: period:this_year,state:open. The available options for filtering are:
all, open, pending_payment, paid or cancelled
This can either be the description of a period (this_month, prev_month, next_month, this_quarter, prev_quarter, next_quarter, this_year, prev_year, next_year) or a custom period (201301..201302, 20130101..20130131)
true or false
You can filter by multiple states at the same time as well. To do this, separate the state values by a pipe: state:open|pending_payment.
Example: returns all purchase transactions of an administration